U.S. patent application Ser. No. 11/691,421, filed Mar. 26, 2007, and published as Publication No. US 2007/0224348 A1 (“the '421 application”), which is incorporated herein by reference, describes a system and method for ALD in which a flexible substrate is transported back and forth between first and second precursor zones separated by an isolation zone into which an inert gas is injected to inhibit the migration of precursor gases out of the precursor zones.
U.S. patent application Ser. No. 12/980,234, filed Dec. 28, 2010, and published as Publication No. 2011/0159204 A1 (“the '234 application”) which claims priority to U.S. Provisional Patent Application No. 61/290,826, filed Dec. 29, 2009 (“the '826 application”), both of which are incorporated by reference herein, describes a radical-enhanced ALD system in which monatomic oxygen radicals are generated from a compound oxygen-containing second precursor gas, such as CO2, which is not reactive with a first precursor gas, such as trimethylaluminum (TMA). In the system of the '234 and '826 applications, the oxygen radicals are generated from the second precursor at a location upstream from and separated a sufficient distance from the first precursor zone such that the oxygen radicals recombine before migrating into the first precursor zone.
In certain embodiments of the '421, '234 and '826 applications, the substrate may be woven along a serpentine path around rollers or other turning guides spaced apart along the first and second precursor zones, as shown in FIGS. 1, 2, and 4 of the '421 application. This serpentine path configuration results in both sides of the substrate being contacted by the rollers as the substrate moves through the system. The present inventor has recognized that such mechanical contact can interfere with the ALD process, as it may disturb the chemisorbed precursor or result in mechanical damage to the coating and/or underlying substrate. This damage is generally caused by imperfections or particles on the surface of rollers, or by surface imperfections, such as bumps; spikes, general surface roughness, or particles on the surface of the substrate. When such features contact, the thin brittle oxide film may be broken, compromising its barrier properties.
The '421 application describes various turning guides as alternatives to rollers. One alternative described involves sprockets that utilize perforations along the margins of the web, as in camera film reels. Sprockets or other similar turning guides that contact the web along its edges may eliminate contact to most of the web surface. The present inventor has recognized that sprockets and similar turning guides may be difficult or economically impractical to implement when very thin and wide substrate web material is used, as is common in commercial food packaging applications, wherein polymer web on the order of approximately 1-4 meters wide and about 12 microns thick is common. Thin flexible polymer web substrate materials of the kind used for food packaging, having a thickness of less than approximately 25 microns (μm) and a width greater than approximately 200 mm, or thicker substrates having a thickness of approximately 25 to 200 microns and a width greater than about 300 mm, may sag, kink, crease, stretch or misfeed when not supported along substantially their entire width as they pass around the turning guides.
In another alternative described in the '421 application, the turning guides may comprise fluid bearings (e.g., gas bearings) that support the substrate on a dynamic cushion of fluid, such as precursor gas and/or inert gas injected through small perforations in a bearing race of the fluid bearing. However, fluid bearings are complicated and difficult to implement in practice.